Flexographic printing is one of the fastest growing conventional printing processes, with significant commercial application in the packaging and publishing industries.
Flexography involves the transfer of an image from an inked relief plate onto a substrate. The areas on the inked relief plate which contact the substrate, and thereby transfer image characteristics, are known as the “image areas.” Conversely, the “nonimage areas” on the inked relief plate do not contact the substrate. Generally, the image can be transferred to any type of substrate including, without limitation, plastics (such as polyethylene, polyester, and polypropylene), metallic films, cellophane, fabrics, and paper.
Flexographic processes encompass both line art processes and continuous-tone art processes. In line art processes, the image contains only solid black and solid white areas, and does not contain any transitional or “gray” areas. Continuous-tone art processes, by contrast, employ different artistic “tones,” i.e., combinations of light and dark values, to represent a rendered object with transitions between solid black and solid white. Thus, in continuous-tone art processes, black, white, and the various shades of gray in between can be used to express the image.
Generally, continuous-tone art processes involve two steps. First, a scanner or other digitizing device digitizes the subject image. Next, a predetermined algorithm converts the digitized image into halftone dots. These halftone dots, which make up the image areas in the inked relief plate, are varied in size and/or frequency in order to produce the desired tone.
Transfer of the desired image is accomplished by direct contact between the inked relief plate and the substrate. Because of this direct contact, it becomes impractical to represent tones having a lower intensity (i.e., lighter tones) using halftone dots below a certain minimum threshold size. This is because dots below a certain minimum threshold size will not reach the printing surface.
In view of the minimum size limitation for halftone dots, a number of screening techniques are known in the art. These screening techniques include Frequency Modulation (“FM tone modulation”), which involves varying the population or density of halftone dots to express different tones, and Amplitude Modulation (“AM tone modulation”), which involves varying the size of dots to express different tones. Specifically, FM tone modulation employs halftone dots of fixed size and a population (or distribution density) that varies depending on the desired tone. Halftone dots are arranged in pseudo-random fashion on the FM screen grid. In AM tone modulation, spacing of the halftone dots is geometric and fixed. The dots will vary in size depending on the tone represented.
These known screening techniques suffer from several limitations. In FM modulation techniques, single halftone dots become more isolated. This undesirably both increases the printing pressure on the dispersed dots and increases the size of the inked plate relief. Further, in some cases, the transferred image may become distorted as smaller dots no longer reach the substrate. To offset this effect, larger dots are typically used in FM modulation than in AM modulation. This results in “grainy” images, tonal jumps, and discontinuous digital dot gains.
Likewise, AM modulation may not adequately represent image regions having low tonal intensities. This is especially the case where a relief pattern forms the image and non-image areas on the inked relief plate. Here, the very small halftone dots consist of small shapes, which can easily bend, break off, or otherwise distort during the image transfer. The minimum threshold size for halftone dots in AM modulation significantly limits the highlight tonal range.
Known transitional screening processes that mix characteristics of AM and FM modulation do not obviate all of the above identified deficiencies. In one such process, AM modulation may be used above a given population density (known as the transition point). Below the transition point, lighter tones are represented through FM modulation using a low dot frequency. Because of the slight population, greater pressure is applied to each dot during the printing process. This pressure may cause individual halftone dots to break, bend, or otherwise distort. Additionally, some dots below the threshold minimal size may not reach the substrate, thereby failing to transfer a portion of the image. The persisting deficiencies inherent in these known processes often produce undesirable visual distortions.
Basic Flexography Concepts Illustrated
In flexographic printing, the visibility of individual dots is undesirable. The visibility of individual dots is particularly emphasized when dots are positioned on a disordered screen grid. By constraining dot size, the visibility of individual dots is reduced. Thus, dot size is preferably kept as small as possible. Additionally, by constraining the dot size for each tone, the propensity for tonal jumps is reduced. However, as described below, printing stability and the distance between dots devise certain practical limits on dot size.
Referring now to the drawings, FIG. 1 illustrates, through a prior art relief plate 100, the relationship between dot size, dot population, and relief plate depth. A printing surface 110 represents the direct contact point of the substrate with relief plate 100. Relief plate 100 contains three general regions: a solid relief region 120, a high frequency region 130, and a low frequency region 140. High frequency (AM) region 130 and low frequency (FM) region 140 are comprised of a plurality of halftone dots. High frequency region 130 contains a relatively dense population of halftone dots, and has a relatively shallow relief plate depth 135. In low frequency region 145, halftone dots are spaced further apart than high frequency region 130, which results in a relatively deeper relief plate depth 145.
Thus, as the halftone dots become more separated, the depth of the relief plate increases. As the depth of the relief plate increases, so, too, does the minimum size of the halftone dot necessary to reach printing surface 110. A halftone dot 150 is shown below the threshold minimal size, such that it will not reach printing surface 110. Relatedly, as the distance between dots increases, and as the size of the dot decreases, the pressure applied by the printing process may bend a given dot, resulting in distortion. Consequently, the relationship between dot population, relief plate depth, and dot size limits expression of the full range of tonal intensities, especially with respect to the representation of lighter tones. That is, as the population density decreases, the relief plate depth increases, and bigger dots may be needed to reach the printing surface. Because of this phenomenon, larger dots are typically used in FM modulation than in AM modulation screening techniques.
FIGS. 2-4 depict, respectively, orthogonal screen grids for AM modulation, FM modulation, and a transitional technique, all of which are well-known in the art. An AM screen grid 200, depicted by FIG. 2, is comprised of halftone dots on a fixed orthogonal grid. The increasing tone intensity from right to left is accomplished by a gradual increase in dot size in the same direction. The low intensity range shown at the far right of AM screen grid 200 is somewhat constrained, as printing stability (i.e., the propensity for dots to break, bend or otherwise distort) becomes adversely affected below a certain minimal dot size.
FM screen grid 300, shown in FIG. 3, is characterized by “grainy” low intensity tones, tonal jumps, and discontinuous dot gains.
A combination of AM and FM modulation techniques is shown in FIG. 4. Beginning at the far right side of transitional screen grid 400, the lowest tonal ranges are produced by FM modulation techniques. Because FM modulation employs larger halftone dots to achieve lighter tones, concerns regarding dot sizes below a certain threshold are obviated. Moving to the left across transitional screen grid 400, tone intensity increases are achieved, according to FM modulation, by increasing the population density. This continues until a maximum population density is reached at a transition point 410. Beyond transition point 410, additional increases in tone intensity are achieved by AM modulation.
There still exists in the art, however, a need for a halftoning process that produces an improved range of tonal intensities, thereby providing an improved representation of a continuous tone image.